The proposed research would pursue an investigation of the relations between locomotor adaptations, and the structural and functional properties of vertebrate skeletal muscle. As an aid to understanding the characteristics of these correlated systems, emphasis will be placed on studying animals differing in body size. Ultrastructural and biochemical measurements on lizard muscles will examine how the systems responsible for deactivation of skeletal muscle are modified to achieve twitch characteristics appropriate to stride dynamics. Measurements on frogs will determine whether developmental modifications in contractile properties also occur in animals adapted for saltatory locomotion. Both theoretical and empirical studies will be done to determine the characteristics of skeletal muscles contracting against inertial loads. A series of passerine birds of varying body mass will be used to determine the relations between the body-size dependent scaling of wing-stroke kinematics and contractile properties of the pectoralis muscles that the provide the power for flight. In addition to classical contractile studies, I will measure power output of avian flight muscle in repetitive contractions that simulate the in vivo use of the muscles. Overall this study will provide valuable insights into the relations between how skeletal muscles are used during locomotion and the structural and functional properties of these muscles.